A dry thermal printing process for printing a color image to a backing sheet, which thereafter can be utilized to transfer the image to a ceramic substrate.
Processes for preparing xe2x80x9cdecalsxe2x80x9d are well known. Thus, e.g., in U.S. Pat. No. 5,132,165 of Louis A. Blanco, a wet printing technique was described comprising the step of offset printing a first flux layer onto a backing sheet, forming a wet ink formulation free of glass and including a liquid printing vehicle and oxide coloring agent, wet printing the wet ink formulation onto the first flux layer to form a design layer, and depositing a second flux layer onto the design layer.
The process described by this Blanco patent is not readily adaptable to processes involving digital imaging, for the wet inks of this patent are generally too viscous for ink jet printing and not suitable thermoplastic for thermal transfer or electrophotographic printing. Digital printing methodologies offer a more convenient and lower cost method of mass customization of ceramic articles than do conventional analog printing methodologies, but they cannot be effectively utilized by the process of the Blanco patent.
The Blanco patent issued in July of 1992. In September of 1997, U.S. Pat. No. 5,665,472 issued to Konsuke Tanaka. This patent described a dry printing process which overcame some of the disadvantages of the Blanco process. The ink formulations described in the Tanaka patent are dry and are suitable to processes involving digital imaging.
However, although the Tanaka process is an improvement over the Blanco process, it still suffers from several major disadvantages, which are described below.
The Tanaka patent discloses a thermal transfer sheet which allegedly can xe2x80x9c . . . cope with color printing . . . xe2x80x9d According to Tanaka, xe2x80x9c . . . thermal transfer sheets for multi-color printing also fall within the scope of the inventionxe2x80x9d (see Column 4, lines 64-67). However, applicants have discovered that, when the Tanaka process is used to prepare digitally printed backing sheets for multi-coloring printing on ceramic substrates, unacceptable results are obtained.
The Tanaka process requires the presence of two xe2x80x9cessential componentsxe2x80x9d in a specified glass frit (see lines 4-12 of Column 4). According to claim 1 of U.S. Pat. No. 5,665,472, the specified glass frit consists essentially of 75 to 85 weight percent of Bi2O3 and 12 to 18 weight percent of B2O3, which are taught to be the xe2x80x9cessential componentsxe2x80x9d referred to by Tanaka. In the system of this patent, the glass frit and colorant particles are dispersed in the same ink. It is taught that, in order to obtain good dispersibility in this ink formulation, the average particle size of the dispersed particles would be from about 0.1 to about 10 microns (see Column 4 of the patent, at lines 13-17).
In the example presented in the Tanaka patent (at Column 7 thereof), a temperature of 450 degrees Celsius was used to fire images printed directly from thermal transfer sheets made in accordance with the Tanaka process to a label comprised of inorganic fiber cloth coated with some unspecified ceramic material.
When one attempts to use the process of the Tanaka patent to transfer images from a backing sheet to solid ceramic substrates (such as glass, porcelain, ceramic whitewares, etc.), one must use a temperature in excess of 550 degrees Celsius to effectively transfer an image which is durable. However, when such a transfer temperature is used with the Tanaka process, a poor image comprised with a multiplicity of surface imperfections (such as bubbles, cracks, voids, etc.) is formed. Furthermore, when the Tanaka process is used to attempt to transfer color images, a poor image with low color density and poor durability is formed. The Tanaka process, although it may be useful for printing on flexible ceramic substrates such as glass cloth, is not useful for printing color images on most solid ceramic substrates.
It is an object of this invention to provide a digital process for preparing a decal for transferring images to a ceramic substrate, which can effectively and durably transfer an image with improved optical properties.
In accordance with this invention, there is provided a digital process for preparing a ceramic decal. In one step of this process, a digitally printed ceramic colorant image is applied to a coated backing sheet utilizing a specified colorant. A flux covercoat is digitally printed to the assembly either before or after the ceramic colorant image has been applied; the covercoat contains both frit and binder, and the frit has a melting temperature of at least 550 degrees Centigrade. The total amount of frit applied to the backing sheet in this process is at least 2 times as great as the total amount of colorant used in the process.